This actually happened to me like 2 years ago. First time was a letter in the mail from Comcast stating that I should check my [wireless] network security, if I use wireless, and enable WEP/WPA so that other people cannot leech off my broadband connection. The second time, they put my cable modem on "abusive mode" which gave it an internal ip address for the comcast network. I think at this point, I was only able to go to the main comcast web page. I had to call them up and release the restriction, but they told me that if I exceed the aggregate bandwidth transfer cap/limit/number, then I would have to find another isp.
Now, Comcast offers tiered service: standard 4/384, 6/384, and 8/768. I would assume that purchasing the higher tiers would in essence increase the aggregate transfer limit cap by that fraction. There was a massive download I wanted totaling 80gig by itself, and before I downloaded it, purchased the highest tier. The 3rd tier is essentially double that of the standard service, so if 90gigs a month was the magic number, then my new cap should be 180gigs a month. Ever since then, I haven't had a problem with my transfers (knock on wood!).